Noise reducing means for cabin aircraft



July 5, 1938.

S. J. ZAND NOISE REDUCING MEANS FOR CABIN AIRCRAFT Original Filed March 29, 1933 57 INVENTOR QTEPHE/V ..T. ZIND.

' ATTO NEY.

Patented July 5, 1938 NOISE REDUCING MEANS FOR CABIN AIRCRAFT Stephen J. Zand, Forest Hills, N. Y assignor to Sperry Gyroscope Company, Inc., Brooklyn, N. Y., a corporation of New York Application March 29, 1933, Serial No. 663,302 Renewed August 21, 1937 2 Claims.

v This invention relates to noise reducing means for aircraft, especially of the cabin type designed for carrying passengers. One of the principal objections of the general public to aircraft travel has been the excessive vibration and noise in the passenger cabins, rendering conversation impossible and contributing largely to general discomfort and air sickness. While many schemes have been proposed for sealing the cabin so that extraneous sounds do not enter the same, prior designers have usually overlooked the fact that a large amount of engine noises enter through the ventilating and/or heating passages employed. One means of accomplishing a reduction in noise is to prevent the greater portion of the engine noises from entering through the ventilating and heating passages by specially tuned sound filters.

Another means is to reduce the noise emitted from the engines themselves, especially in multimotored craft without decreasing the efficiency thereof. In such craft it is found that several engines usually have very close periods of vibration'so that the predominant sound from each is a note or less apart. Such a combination produces very disagreeable beat notes, or in other words discords. To eliminate this I provide means for tuning the predominant notes of the engines so that they are the same or some nonbeat producing relative frequency. By this means the dominant notes or sound periods emitted from the two engines are made to harmonize. In addition, by interference effects and proper filtering the total sound may be greatly reduced.

Referring to the drawing Fig. l is a plan view, partly in section, of a cabin airplane showing the ventilating passages.

Fig. 2 is a section through one of the ventilating passages showing one form of sound filter therein.

Fig. 3 is a sectional detail of one of the individual ventilators.

Fig. 4 is a sectional view of a heating passage adapted to conduct warm air from the engine with a different form of sound filter.

Fig. 5 is a horizontal sectional view through the cabin below the section of Fig. 1 and showing the heating pipes extending along-the fioor.

Fig. 6 is a sectional detail of a modified form of that portion of the invention relating to engine tuning.

Fig. '7 is a top plan view of t e filter shown in Fig. 4.

Fig. 8 is a sectional detail of one of my variable exhaust pipe nozzles for tuning purposes.

I prefer to take in the ventilating air at a point remote from the engines 24 and 25 so as to take as little noise as possible into the ventilating system. For this purpose I may place the air intake 1 into the nose of the cabin, which in a twin engined ship lies some distance ahead of the engines and propellers. Said intake is connected to ventilating passages 2 and 3 extending along the sides of the cabin, preferably above the heads of the passengers. Said passages are preferably of tapering cross section to equalize the air fiow from the several ventilating nozzles 4, 5 and 6 and are preferably lined with some sound absorbing material, such as soft felt or wool. It should be noted that the first nozzle or outlet 4 is placed some distance from the intake I so that there is sufficient area or length of felt surface between the outlet and the intake to absorb most of the sound entering at I. Even with such a lining, however, some of the roar of the engines and propellers may pass into the plane and I, therefore, may provide additional means, preferably in the form of a sound filter l to attenuate the predominant sound frequencies of the craft. Such a filter may be in the form of an opening 8 in the wall of the tube 2 and having a short pipe 9 connected thereto. Said pipe has a small hole or holes it in one end thereof communicating with the atmosphere within the cabin and may be of adjustable length as by having the sleeve ii slidable upon the cylindrical portion l2 so that the filter may be adjusted not to pass the predominant sound frequencies of the craft. 'Ijhus, by telescopically adjusting the sleeve II on pipe 9, the internal volume of the filter is correspondingly varied so that the audible sound waves emitted through holes It] may be made to be out of phase with respect to those entering the cabin through nozzles or ports 4, 5 and 6 by as much as desired, to thereby effect the substantial nullification of the sound waves, one by the other. Such a filter is preferably placed in both air ducts 2 and 3 by the use of which the noise entering the craft through such ducts is very greatly reduced.

Aircraft are usually also provided with warm air ducts l3 for heating the craft. These may extend under the seats 40 and along the floor at the sides of the cabin. Such ducts usually obtain the air supply from a hood l4 placed around a portion of the exhaust pipe or manifold [5 of the engine, air entering the hood being warmed by the exhaust pipe and then passing through the connecting pipe l6' to the duct l3, which may have a plurality of apertures therein (not shown) under the seats 40 of the passengers. Such a duct I also preferably line with sound absorbing material which, in this case, is preferably also the proof, such as glass wool or asbestos wool 18 and either in the pipe IE or in the duct 13 or both I place sound filters II (II') and 31 corresponding in function to the sound filters I.

A variation in the form of such filters is shown in Fig. 4 in which the size of the aperture i9 therein connecting with the atmosphere is varied by making the aperture of triangular shape and. having slidably mounted thereover a plate 20 to cover more or less of the aperture as shown in Fig. 7.

I also find it possible to reduce the disagreeable beat notes of multi-engined planes described above by some method of tuning or synchronizing the predominant sound frequencies caused by the engine exhaust. To this end I may provide at least one of the exhaust pipes IS with variable end or nozzle plates 22 having different size holes 23 by which the predominant note of the exhaust may be varied so as to properly harmonize with the predominant note of the other .engine and, therefore, avoid the objectionable beat notes referred to. Another method I have shown is to provide a cross pipe 2i connecting the exhaust pipes l5 and 15' of the two engines. Such a pipe is preferably provided with a small opening 26 to the atmosphere which may have a short pipe 21 connected thereto, the opening of which is adjustable by a needle valve 28. By such a means the disagreeable'beat notes above referred to may be eliminated either by interference or filtering or both and the general sound level of the engines greatly reduced.

When an aircraft cabin is equipped with my invention as outlined above and in addition is suitably insulated from vibration as explained in my copending application, Serial No. 666,940 filed April 20, 1933, the greater part of the cabin noise of aircraft is eliminated so that the sound level is about the same as that of a pullman car, i. e., about '75 decibels instead of on the order of 105 decibels which is common in multi-motored planes and at which sound level it is next to im possible to converse.

In accordance with the provisions of the patent statutes, 'I have herein described the principle and operation of my invention, together with the apparatus which I now consider to rep? resent the bestemliodiment thereof, but I desire to have it understood that the apparatus shown is only illustrative and that the invention can be carried out by other means. Also, while it is designed to use the various features and elements in the combination and relations described, some of these may be altered and others omitted without interfering with the more general results outlined, and the invention extends to such use.

Having described my invention, what I claim and desire to secure by Letters Patent is:

1. In a noise reducing ventilating system for an aircraft having an engine driven propeller and a closed cabin, a ventilating channel extending within the aircraft cabin and opening to the exterior of the cabin at the forward portion thereof, said channel having ventilating ports at points along the length thereof within said cabin, and a sound filter within said cabin communicating with said channel, said sound filter comprising a container arranged to have its volume varied at will and apertured for providing sound communication between the interior thereof and said cabin, the volume of said container being set in use so that the sound waves communicated therefrom into the cabin are out of phase with the sound waves entering the cabin through said ventilating ports, whereby the sound waves emitted from said filter substantially nullify those emitted from said ports.

2. In a noise reducing ventilating system for an aircraft having an engine driven propeller and aclosed cabin, a ventilating channel extending within the aircraft cabin and opening to the exterior of the cabin at the forward portion thereof, said channel having ventilating ports .at points along the length thereof within said cabin, said channel being of tapering construction so that the cross section thereof is gradually reduced from one ventilating port to the next such port rearward thereof, a sound absorbing iining provided on the interior surfaces of said channel, and a sound filter within said cabin and connected to said channel, said sound filter comprising a telescopical container provided with small apertures to enable a limited volume of audible sound waves to enter the cabin therefrom, said container being adjustably telescoped in use so that sound waves communicated therefrom into the cabin are out of phase with the. sound waves entering the cabin through said ventilating ports, whereby the sound waves emitted from said filter substantially nullify those emitted from said .ports.

STEPHEN J. EiANlIi. 

